SELECTIVE OXYGENATION OF HYDROCARBONS AND SULFOXIDATION OF THIOETHERS BY DIOXYGEN WITH A MN-PORPHYRIN-BASED CYTOCHROME P450 MODEL SYSTEM USING ZN AS ELECTRON-DONOR

Cyclooctene epoxidation by O2 OCCUrs, with consumption of reducing equivalents from Zn and protons from acetic acid, in the presence of two catalysts, Mn(TPP)Cl and 1-Melm. A comparative study made on ten different Mn(III) or Fe(III) porphyrin catalysts, ten nitrogen base cocatalysts and various carboxylic acids led us to select the most efficient system which involves Mn(TPP)Cl, 1-Melm and CH3COOH in addition to O2 and Zn in a CH3CN:CH2Cl2 mixture. This system selectively epoxidizes 2-methylhept-2-ene, cyclohexene, (+)limonene, cis-stilbene and alpha-ionone with yields based on Zn between 34 and 58% and rates between 1 and 3 turnovers per min. It also epoxidizes 1-nonene, a less reactive alkene, and oxidizes alkanes like cyclooctane, cyclohexane, adamantane, indane, tetralin and heptane to the corresponding alcohols and ketones, but with lower yields (between 1 and 36%). Thioethers are selectively oxidized to the corresponding sulfoxides with yields up to 68%. The system exhibits a stereochemistry for cis- and trans-stilbene epoxidation, a regioselectivity for the oxidation of cyclohexene, limonene and heptane, and a chemoselectivity for the oxidation of a cyclooctene-cyclooctane mixture, almost identical to those of the Mn(TPP)Cl-PhlO-1-Melm system. This indicates that the Mn(TPP)Cl-O2-Zn-AcOH-1-Melm system involves a (1-Melm)Mn(V)=O active oxygen species. This system was successfully used for the conversion of alpha-ionone to the corresponding epoxides and allylic ketone and of dibutylthioether to its sulfoxide with high yields and without any denaturation of the catalyst.